Telecommunications system

ABSTRACT

An asynchronous transfer mode adaptation layer switching network has independent call routing and connection control for setting up connections across the system. A plurality of adaptation layer switches (ALS) coupled to the ATM network comprise a group adapted to function as an adaptation layer switching network whose fabric and control are distributed over the group. The network having means for determining its current system status whereby to set up multimedia calls across the network based on that status determination.

This invention relates to digital communications systems and inparticular to systems embodying asynchronous transfer mode (ATM)technology. The invention further relates to the application of PNNI(Private Network Network Interface) signalling to control an ATMadaptation layer switching network.

BACKGROUND OF THE INVENTION

The asynchronous transfer mode (ATM) technology is a flexible form oftransmission which allows any type of service traffic, voice, video ordata, to be multiplexed together on to a common means of transmission.In order for this to be realised, the service traffic must first beadapted typically into 53 byte cells comprising 5 byte headers and 48byte payloads such that the original traffic can be reconstituted at thefar end of an ATM network. This form of adaptation is performed in theATM adaptation layer (AAL).

A recent development has been the introduction of the AAL-2 adaptationlayer. This adaptation layer has been optimised to accommodate thedemands of low bit-rate communications representing the increasing trendto greater voice compression. The adaptation layer is a multiplex ofusers in a single ATM connection where each user's information iscarried in short packets or minicells each with a header identifying theuser channel and incorporating ancillary control information. Thisconstitutes a dynamic trunk group of users in a single connection.

As telecommunications networks increase in complexity and carryincreasing volumes of traffic, the current procedures for setting upconnections between subscribers are limiting the performance of thesenetworks. In particular, congestion may be caused by attempting toconnect to a subscriber who is already busy, or by attempting to choosea route through an already congested part of the network. Thus equipmentand resources can be wasted in attempts to set up calls which cannot becompleted. A further problem is that of scalability. As the networkexpands to accommodate increased traffic and a larger number ofsubscribers, there is an increasing need to facilitate integration ofnew equipment into an existing network without simply increasing thecongestion problem. Moreover, careful planning is required to ensurethat calls can be routed through the network. This requires an ad-hocdistributed routing decision policy which limits the flexibility of therouting process as the call routing must be collocated with theswitching node fabric.

SUMMARY OF THE INVENTION

The object of the invention is to minimise or to overcome thesedisadvantages.

A further object of the invention is to provide an improved method ofoperating a telecommunications network.

According to one aspect of the present invention there is provided aasynchronous transfer mode adaptation layer switching network havingindependent call routing and connection control for setting upconnections across the system.

According to another aspect of the present invention there is provided adistributed telecommunications exchange system having means fordetermining at a call source the current status of the system whereby toeffect routing of a multimedia call across the system.

According to another aspect of the invention there is provided a methodof communicating resource availability to maintain performance of anasynchronous transfer mode adaptation layer switching network underoverload conditions.

According to a further aspect of the invention there is provided amethod of routing telecommunications traffic in a system including anasynchronous transfer mode (ATM) network having uncommitted bandwidth,and a plurality of adaptation layer switches (ALS) coupled to the ATMnetwork, which adaptation layer switches comprise a group adapted tofunction as an adaptation layer switching network whose fabric andcontrol are distributed over the group, the method including determiningthe current system status whereby to set up multimedia calls across thenetwork based on that status determination.

We have found that adaptation layer switching can be incorporated intoan ATM private network network interface (PNNI) reference architectureto provide a mini channel connectivity layer and a routing architecturefor establishing connections in the adaptation layer.

The arrangement and method facilitates both scalability of the networkand the separation of call routing and connection control. The advantageof this separation of the two functions can be exploited by makingrouting decisions at the outset. Dynamic trunking allows the routingdecisions to be independent of the connection control. With fixed trunknetworks, voice routes are made up of small trunk groups that pertain tophysical links which may be diversely connected. The status of theseconnections will affect the final route available. Because the status isdistributed, so must be the routing, and hence there can then be noseparation.

The arrangement and method further facilitate the simplification of thePNNI model to reduce hierarchy, obviate crank back and alternate pathrouting, and simplify call admission policy on a service related basis.

PNNI may be applied to dynamic trunking in the adaptation layer usingATM connections to represent the physical bearer, but additionally thevirtual connectivity allows dual homing, load-balancing, diverse routingand virtual connections may be re-established automatically overalternative physical paths to maintain connectivity. This enables theconstruction of an extremely robust network.

Reference is here directed to our co-pending application No.GB-9614138.7. This application describes a telecommunications systemcomprising an asynchronous transfer mode (ATM) network havinguncommitted bandwidth, and a plurality of adaptive grooming routers(AGR) coupled to the network. The AGRs comprise a group adapted tofunction as a virtual transit exchange whose fabric and control aredistributed over the group. The virtual transit exchange comprising theAGRs incorporates independent connection control and call routingfunctions and has means for determining the current system statuswhereby to set up narrow band connections across the ATM network basedon that status determination.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 illustrates the multiplexing of minicells into ATM;

FIG. 2 is a schematic diagram of an ATM adaptation layer switchingnetwork illustrating end to end minicell connection and traffic flow;

FIG. 3 illustrates the architecture of an adaptation layer switchemployed in the network of FIG. 2; and

FIG. 4 illustrates routing in the network of FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring first to FIG. 1, this illustrates the way in which AAL-2minicells are multiplexed into ATM. The AAL-2 adaptation layer has beenoptimised to cope with the demands of low bit-rate communications,representing the increasing trend to greater voice compression. Theadaptation layer is a multiplex of users in a single ATM connection,where each user's information is carried in a short packet or minicells,with a header identifying the user channel with ancillary controlinformation (see FIG. 1). This is a dynamic trunk group of users in asingle connection, and thus the routing techniques described in ouraforementioned co-pending application No. 9614138.7 may be applied. Eachminicell has a service specific payload and a three octet headercontaining typically the channel identifier, the cell length and theSSCS control. The minicells are multiplexed into ATM cells each of whichis provided with an ATM header. Where appropriate, dummy minicells areinserted as padding in the ATM cells.

By sharing the fixed length payload of the ATM cell between users, thecompromise of trading cell assembly delay for bandwidth efficiency isneatly side-stepped, a sacrifice which would be acute at low bit-ratesand on expensive leased lines. AAL-2 adaptation equipment performs aconcentration function to ensure high utilisation, but can also limitthe holdover delay of traffic when usage is low.

A further feature of minicells is that they may be of variable size,from 1 to 64 octets, to accommodate a wide variety of applications withminimal overhead. Thus the mapping to ATM cells is asynchronous and infact quite independent of the length of an ATM cell. The boundary ofminicells in the ATM cell payload is signified in every cell by a startfield (STF), which specifies the offset, and thus minicells form aself-delineating flow.

Minicells provide a universal adaptation medium able to support voice,video and data in a common ATM VCC. In the access segment of thenetwork, such a connection can be carried transparently over videodistribution systems using MPEG transport stream, narrow band systemswith 64 kb/s capacity, n×64 kb/s, or a modem channel without any loss offlexibility or efficiency. The result is a multimedia service transportwhich is transparent to practically all physical transport systems, butwhich is at the same time fully integrated into ATM.

Referring now to FIG. 2, this illustrates end to end flow of trafficbetween network adaptors 11 a, 11 b via adaptation layer switches 12 a,12 b. The AAL-2 standard specifies a feature that allows minicells to berelayed between connections, without the need to terminate the carriedservice as illustrated in FIG. 2. This provides the ability to establishand control a minicell channel (adaptation layer) connection across manynodes forming an adaptation layer switching network.

The set-up, maintenance and clear-down of minicell channels in an AAL-2connection, and the switching operation of the relay, is controlled bythe AAL-2 Negotiation Procedures (ANP). AAL-2 connections are supportedby in-band F7 OAM minicells, replicating for the adaptation layer theconnection maintenance capabilities of ATM.

An adaptation layer switching network and node architecture is shown inFIG. 3, highlighting the adaptation layer nodes. The connectivity isprovided by Switched and Permanent VCs (SVC/PVC) in the ATM layer, whichhas not been shown, but behaves as a virtual connectivity backplane. InFIG. 3, the nodes have been arranged and annotated using the ATM ForumPNNI notation for the description of peer groups, which are a convenientform of routing abstraction to form a Logical Group Node (LGN) fromphysical nodes or other LGNs in a hierarchical manner.

For this adaptation layer technology, only two layers of hierarchy arenecessary to build a massively scaleable network. The lowest layer isthe Adaptation Layer Switch (ALS) which is an LGN formed from physicalports and a single stage distributed core, and may represent collocated,and commonly controlled and managed physical devices. Similarly, EdgeDevices (ED) not shown in detail, share much common technology with theALS, with the addition of a service specific interworking function. Thetop level is a network level peer group comprising a symmetrical andanalogous meshed connection of ALSs and Edge Devices.

Using AAL-2 VLSI technology described later, ALS nodes can support ofthe order of a million 64 kb/s circuits with today's technology, and anorder of 10s of millions of low-bit-rate voice connections. Thus thenetwork shown with a single switching stage can scale easily to globalproportions. More hierarchical levels and switching stages can be usedwith no loss of applicability of the routing and congestion controllingmechanisms. However, the simplicity and symmetry of the network shownallows straightforward congestion management by load-balancing andrejection of blocked traffic at source. This yields the highestbandwidth utilisation, from a minimum sufficiency of ALS nodes, to meetany QoS targets on call blocking. The ALS can be designed to be a fullynon-blocking node to improve throughput, and ensure stable behaviourunder overload conditions.

The advantages of scaling and low blocking probability are achieved byAAL-2 connections effectively operating as Dynamic Trunk Groups (DTG).These correspond to virtual trunk groups of an adaptive grooming router.An AAL-2 connection is a multiplex of users in a virtual channel, thathas a concentrating function to ensure high utilisation, but also hashigh efficiency since the connection can be configured as aDeterministic Bit Rate (DBR) service and share a very large physical orvirtual pipe.

The completed ATM Forum PNNI phase 1 specification allows for thedynamic behaviour of a network; changing link state parameters can becarried in PNNI Topology State Packets (PTSP), for example to informnodes within the same peer group of a change in available bandwidth. Inthe single stage network of FIG. 3, knowledge of the available bandwidthend-to-end is easily obtainable, and permits the ultimate success ofrouting any given call, to be assessed at the source ED from arelatively up-to-date view of congestion in the network.

With regard to monitoring dynamically changing bandwidth, the PTSPs canbe transmitted with fixed periodicity, or by crossing thresholds ortriggers, optimised to achieve highest network bandwidth utilisation andnear zero call blocking probability under normal load, for a minimaloverhead. This is equivalent to the advertisement procedure of ouraforementioned co-pending application No. 9614138.7. PNNI Parameterssuch as Mean Available Cell Rate (Mean ACR) and a link administrativeweight for route priority can be used to load balance traffic to furtheravoid localised congestion. Load-balancing and the simplicity andsymmetry of the network, diverse logical or physical path routingcombined with PNNI soft PVCs, ensure a resilient network can be built.

The ED can apply both a service related and the PNNI Generic CallAdmission Control (GCAC), by determination of the route, and byprediction based on available bandwidth—knowledge of both it has at theeffective edge of the network. An analogous procedure is described inour aforementioned co-pending application No 9614138.7. From this the EDcan determine the probability of successful call establishment, suchthat blocked traffic can be rejected at source. In PNNI terminology,this route determination results in a Designated Transit List (DTL) thatrequires no further routing decisions at the intermediate ALS nodes. TheED's complete view of the network means that the crank back andalternate path routing of PNNI need not be implemented. The ALS physicalnodes can run a PNNI Actual Call Admission Control (ACAC) to ensure thatcarried traffic does not exceed particular criteria, and to correctlyintercept race conditions. This corresponds to grooming or trunkingcapacity of an adaptive virtual junctor or adaptive grooming router nodein our aforementioned co-pending application No 9614138.7. The residualvery low probability of blocking means that a blocked call can bereleased back to the source, rather than attempt alternatives within thenetwork.

FIG. 4 depicts the adaptation layer connection establishment mechanismwhich is a straightforward extension of the PNNI protocol. The AAL-2standard specifies a network architecture that comprises adaptationnodes that are the Edge Devices (ED) and relay nodes that is the ALSshown in FIG. 4. As mentioned earlier, the standard also specifies aconnection management protocol called Adaptation Negotiation Procedures(ANP).

The ANP can operate on a link-by-link basis, but can also be cascaded toprovide end-to-end connectivity in a robust manner. The figure shows atwo-state Finite State Machine (FSM) at both the transmitter andreceiver, which corresponds to one AAL-2 VCC. The ANP protocol withregard to the set-up and release of minicell channels, is basically a“request-response-action” handshake protocol.

To set-up a minicell channel the transmitter, normally in the idlestate, sends a request to the receiver, and by that action it hascommitted itself to effect the change. Consequently the transmitterexpects a response. It will retry at certain intervals in case theresponse was lost, before deeming a receiver failed so that thetransmitter can simply reclaim the channels in the AAL-2 VCC to thatreceiver. A response of acceptance or denial takes the transmitter backto the idle state.

The receiver operates in an analogous manner, but is slaved to thetransmitter's request messages. When a request is received, the receiversends a response and moves out of the idle state. The receiver's normalexpectation is that its affirmative response will be accepted by thetransmitter, which is already committed. Therefore the receiver commitsto providing resources for the channel. A transition back to the idlestate is effected when a confirmation is received explicitly, orimplicitly by receipt of the first minicell with the new channel or anin-band indication. If no form of confirmation is received, and arelatively long time-out has expired, the receiver can deem thattransmitter to have failed and reclaim any resources it used.

The PNNI protocol determines the routing as a Designated Transit List(DTL) as described earlier, and this is provided as an InformationElement (IE) in the originating Q.2931 set-up or add party messages thatwould normally invoke SVC set-up in the ATM layer. This routingdetermination, which corresponds to the route or “worm” of ourco-pending application No 9614138.7, is easily adapted to AAL-2technology. Signalled directly, Edge Devices can extract the IE fromQ.2931 and use it directly in an ANP set-up message. This constitutesconnection control and provides the routing description to direct thecascade of minicell channel connection establishment between adaptationlayer nodes. Consequently the ATM network can be configured using PVCs,although this does not exclude the use of SVCs and thus guarantees thegrade of service.

Logical nodes with aggregated hierarchy such as the ALS in FIG. 4 canadd nested levels of DTL to the IE to route across their collocatedphysical modules in an equivalent manner. The robustness of the singleANP stage allows a pipelined forwarding of the IE, and a single point ofentry means that the network operates as one logical switch.

Connection control can establish from the outset those voice routeswhich are not reachable due to internal congestion. By knowing thecapacity available on the AAL-2 connections, because these can support adynamically varying number of calls, connection control knows fromcongestion criteria whether it can dilate the bandwidth of anyparticular AAL-2 connection and what the greatest likelihood of successwill be for any given choice. This provides a facility to block routeswhich are unreachable in the network at the outset. By rejecting callswith as small amount of processing as possible then, when the system isheavily overloaded there is reduced processing and signalling generatedfor calls which can not currently be established across the network. Byproviding this essentially negative feedback scheme, a call can berejected at the periphery of the ALS network before an abortive attemptto route the call across the network and thus a high degree of carriedtraffic can be maintained. Connection control can achieve a loadbalancing capability with voice routes which are of equal priority.

The AAL-2 connections could be given for example a PNNI LGN designator,or a geographic location, or reach or distance indicator, and therebythe congestion could be sent out only to a restricted set of nodes forexample if the distance is below a certain threshold with an associatedcongestion threshold, thereby only ALSs within a given geographic localeor PNNI LGN will get signalled first and as congestion increases then itcould spread to the wider ALS network system. One can envisage anyscheme of selection criteria for any advantageous purpose, which may beselect or universal in application.

The arrangement and method described provide for the distribution of theALS in terms of its fabric and its control and its enabling technologieswith reference to dynamic trunking concepts. There is uncommittedbandwidth within the fabric and consequently routing decisions can befully independent of the operation of this distributed fabric. Thedistributed fabric can, because of this unallocated bandwidth useseparate connections to the control layer for establishing theconnections through the fabric and this in no way compromises externaldecisions that are made except when an overload situation isencountered. An advertising process using PNNI PTSPS provides knowledgeof the distributed fabric. This includes local knowledge about a remotesite such that routing decisions can be made and modified wherenecessary so as to reject traffic at source. The dynamic trunkingenables the separation of call routing and connection control. Thearrangement also provides a means of ensuring stability under overloadsituations and minimising the cost of handling traffic which would berejected by destinations.

The separation of call routing and connection control together withadvertising the system status ensures a wide range of scaleability sothat the application of dynamic trunking technology providesscaleability in a traffic sense. Further, the separation of call routingand connection control provides a distributed computing environmentwhich is scaleable and managed by this advertisement resource. Becausethe distributed exchange manages its own internal traffic, effectivelyit provides means for balancing that traffic to the fabric and makes itsown internal routing decisions. The separation further provides afacility to support a wide range of services using other signallingschemes. The fabric provides a connection engine that can accommodate awide variety of signalling protocols appropriate for the type of serviceto be provided and that can set up connections.

The full knowledge of the network connectivity and the release ofresources can be enabled by any node within the network, as it can tracethrough all connections from any starting point. This can be used tosupport failure recovery.

It will be appreciated that although the arrangement and method havebeen described above with particular reference to current standardprotocols, it is in no way limited to the use of these particularprotocols.

It will be understood that the above description of a preferredembodiment is given by way of example only and that variousmodifications may be made by those skilled in the art without departingfrom the spirit and scope of the invention.

What is claimed is:
 1. A method of routing telecommunications traffic inan asynchronous transfer mode (ATM) adaptation layer switching networkcomprising a plurality of nodes arranged in peer groups and havinguncommitted bandwidth, there being a plurality of adaptation layerswitches (ALS) coupled to the ATM network, which adaptation layerswitches comprise a group adapted to function as an adaptation layerswitching network whose fabric and control are distributed over thegroup, the method including determining the current system status andbandwidth availability, and effecting routing decisions at the networkedge consequent on said bandwidth determination whereby to set upmultimedia calls across the network.
 2. A method as claimed in claim 1,wherein said routing is effected via a Private Network Network Interface(PNNI) signalling protocol.
 3. A method as claimed in claim 2, whereinchanging link state parameters are carried within PNNI topology packetswhereby to inform a set of network nodes within a peer group of a changein available bandwidth.
 4. A method as claimed in claim 3, wherein thePNNI protocol controls the adaptation layer of the network.
 5. A methodas claimed in claim 4, wherein route determination between network nodescomprises a designated transit list (DTL) that requires no furtherrouting decisions at the intermediate nodes.
 6. A method as claimed inclaim 5, wherein the route determination proceeds in a hierarchicalmanner according to the PNNI protocol, wherein the network nodescomprise hierarchical logical groups, wherein a designated transit listdetermines the route between designated network nodes, and wherein therouting within a said logical group is determined on entry to thatgroup.
 7. A method as claimed in claim 6, wherein the designated transitlist is provided as an information element in an originating set-up oradd-party message for invoking SVC set-up in the ATM layer.
 8. A methodas claimed in claim 6, wherein call admission is effected on a servicerelated basis.
 9. A method as claimed in claim 6, wherein saidadaptation layer is an AAL-2 adaptation layer.
 10. A method as claimedin claim 9, and including determining availability of a destination, andrejecting at source traffic to that destination in the event that thedestination is unavailable.
 11. An asynchronous transfer mode (ATM)adaptation layer switching network comprising a plurality of nodesarranged in peer groups and having uncommitted bandwidth, and aplurality of adaptation layer switches (ALS) coupled to the ATM network,which adaptation layer switches comprise a group adapted to function asan adaptation layer switching network whose fabric and control aredistributed over the group, the network having means for determining itscurrent system status and bandwidth availability, and means foreffecting routing decisions at the network edge consequent on saidbandwidth determination whereby to set up multimedia calls across thenetwork.
 12. A network as claimed in claim 11, and having means fordetermining availability of a destination, and for rejecting at sourcetraffic to that destination in the event that the destination isunavailable.
 13. A network as claimed in claim 11, wherein the routingmechanism for determining the availability of a destination and forrejecting traffic to that destination as source is independent of thescale of the network.